Added git functions - "Merge" and "Rebase"
They are in the "Branches" dialog:
- Merge - merge selected branch into current one
- Rebase - rebase current branch on selected one

Task-number: QTCREATORBUG-8367

Change-Id: I9ed306c64d5d4b7bd1d58730a5e1009f0bd4ec0e
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>